The present invention relates to an optical fiber device such as an optical fiber laser or an optical fiber amplifier. The invention is particularly suitable for use as a fiber amplifier in a multi-fiber cross-connect in an optical communications system.
Conventional Erbium-doped fiber amplifiers use single mode, single clad fiber, pumped with single mode pumps. Recently, high power amplifiers have become available which use double-clad fibers. High power multi-mode pump-light from an optical source can then be coupled into the inner cladding of the optical fiber. In both cases, interactions with the doped core of the optical fiber cause excitation of the atoms of the dopant material. As an optical signal propagates along the length of the fiber, stimulated emission occurs as the excited Erbium dopant decays, thereby amplifying the passing signal.
As mentioned above, the present invention is not limited to fiber amplifiers and also applies, for example, to optical fiber lasers. These are similar to fiber amplifiers in construction, although reflectors are additionally included arranged at positions along the optical fiber to define a laser cavity in the optical fiber so as to generate a laser output when the fiber is pumped.
In order to couple pump light into the cladding of the optical fiber, dedicated fiber couplers (usually one per fiber) are required, which introduce loss to the system and result in significant cost. Multi-mode pump lasers can be used with double clad fiber. These allow higher power multimode lasers to be used (which may be multiplexed using a bundle), but one semiconductor laser is required per fiber laser or amplifier.
There is increasing interest in all-optical switching devices for use in nodes of optical communications networks. These devices comprise optical cross connects, which enable a large number of input signals to be routed selectively to a corresponding number of outputs. Each input and output is carried by a respective fiber, and the signal on each fiber is preferably amplified to compensate for attenuation which occurs in the cross connect. 1024xc3x971024 optical cross connects are being developed, requiring a large number of amplifiers. The need for dedicated fiber couplers adds substantial complexity and cost. There are numerous other applications where a large fiber bundle is present, with the signals on each fiber requiring amplification. These include cable television applications.
According to the present invention, there is provided an optical device, comprising:
an optical housing having a diffusive reflective inner surface and at least one opening for receiving pump light;
an optical fiber having a doped core at least partially contained within the optical housing.
The invention provides an optical device which enables an optical fiber to be pumped by an optical source without the need for any complex or expensive optical couplers. Pump light is provided to the doped core by uniform illumination around the fiber rather than by an end feed arrangement. This lateral illumination enables many fibers to be pumped by the same pump source, so that in multi-fiber applications, a separate dedicated coupler is no longer required to couple the pump light into each optical cable. Preferably, therefore, the device comprises a plurality of doped optical fibers a part of each of which is contained within the optical housing. This enables the plurality of optical fibers to be provided with pump light simultaneously, without requiring individual couplers. A low cost high power diode may be used to pump the multiple amplifiers simultaneously. The cost and complexity of the multi-fiber pumped optical systems can be greatly reduced. The invention also enables multimode semiconductor pump lasers to be employed without requiring double-clad fibers to be used.
In one example, the device is an optical fiber laser, further comprising a first reflector arranged at a first position along the optical fiber and, a second reflector arranged at a second position along the optical fiber wherein the optical fiber between the first and second reflectors defines a lasing cavity of the laser and wherein at least part of the lasing cavity is contained within the housing, the second reflector being a partial reflector to allow a laser output from the fiber laser.
In another example, the device is an optical amplifier suitable for use in amplifying the signals in a large number of optical fibers simultaneously. This enables amplification to be provided cheaply and conveniently to the signals in fibers at an optical cross-connect.
Preferably, the device further comprises an optical source arranged at the opening of the optical housing to provide pump light to the optical housing. Any suitable device may be used as the optical source. Examples include a high power (of the order of Watts) multi-mode diode or multi-mode diode bar laser. These devices are low cost and power efficient.
In one example, the optical housing is a cylinder having substantially parallel upper and lower surfaces and a connecting surface connecting the upper and lower surfaces, and in which the inner surface of the connecting surface has a diffusive reflective coating. Alternatively, the optical housing may be a sphere or any other suitable integrating geometry. The cylinder configuration enables a large number of amplifiers to be provided in a relatively small volume.
According to a second aspect of the present invention, there is provided an optical communications system, comprising an optical cable having a plurality of optical fibers and one or more optical amplifiers arranged at positions along the optical cable, wherein at least one of the optical amplifiers comprises an optical housing having a diffusive reflective inner surface and at least one opening for receiving pump light, wherein a plurality of doped optical fibers are at least partially contained within the optical housing, the doped optical fibers being coupled to the optical fibers of the optical cable.
The benefits of reducing the volume required for amplifying the signals in a multi-fiber communications system enable systems to be designed and built with a greater number of fibers, increasing the overall system information carrying capacity.
According to a further aspect of the present invention, there is provided a method of amplifying a signal propagating in an optical fiber, at least part of the optical fiber being contained within an optical housing, the optical housing defining a volume having a diffusive reflective inner surface, comprising the step of:
providing pump light from an optical source to said optical housing, the pump light reflecting off the diffusive reflective inner surface and substantially uniformly illuminating the volume thereby pumping the optical fiber, to amplify the optical signal.
According to a further aspect of the present invention, there is provided a method of generating a laser output from an optical fiber having a first and second reflector arranged at positions along the fiber defining a laser cavity in the optical fiber, at least part of the laser cavity being contained within an optical housing, the optical housing defining a volume having a diffusive reflective inner surface, comprising the step of:
providing pump light from an optical source to said optical housing, the pump light reflecting off the diffuse reflective inner surface and substantially uniformly illuminating the volume thereby pumping the laser cavity of the optical fiber to generate a laser output.